A poly lactic acid resin is a biomass polymer and therefore has been drawing attention in recent years against the background of the depletion of petroleum resources, the reduction of carbon dioxide emissions, and the like.
However, the poly lactic acid has a low crystallization rate and is unlikely to be crystallized by a common film forming procedure. Hence, a film composed of a resin composition containing the poly lactic acid has a problem of poor heat resistance. For example, such a film is thermally deformed at about 60° C. or more that is a glass transition temperature of the poly lactic acid and cannot keep a film shape.
The addition of a plasticizer and the like is a known method for improving brittleness of the poly lactic acid.
However, commonly, the addition of a plasticizer having good compatibility with the poly lactic acid lowers a glass transition temperature of the poly lactic acid, and the heat resistance of the film is often further lowered than that of a film containing poly lactic acid alone. Thus, such a film is difficult to be used at the glass transition temperature or more.
Patent Document 1 discloses a method for improving the heat resistance in which poly lactic acid including a plasticizer is molded at a temperature between the glass transition temperature and the melting point to improve the heat resistance. However, there is no description about the specific time required for crystallization, and it is unknown whether a film can be continuously formed at a practicable speed.
There is also disclosed a method for obtaining a molded article that has good injection molding properties (time required for molding), a high Vicat softening point (high heat resistance), and flexibility by adding a plasticizer, polyalkylene glycol, and a styrene copolymer to the poly lactic acid (Patent Document 2).
However, the method requires a large amount of the styrene copolymer to remarkably reduce biomass degree. Moreover, such a molded article has a low heat resistance at about 80° C. and does not have a heat resistance applicable for heat-resistant parts in home electric appliances or automobiles.
There is disclosed another method of crystallization by adding a particular plasticizer and a crystal nucleating agent to poly lactic acid to give a mixture, forming a film from the mixture with an extruder, once cooling the film, and then passing the film through a process from 60 to 100° C. (Patent Document 3).
However, the method is inefficient because a film is once cooled and solidified, and then heated again.
There is another problem. That is, when a resin composition containing the poly lactic acid is melted to form a film or sheet using metal rolls, the resin composition adheres to the metal rolls to interfere with the formation of the film or sheet because the resin composition has a poor releasability from the rolls.